Condensed Matter Seminar

The condensed matter seminar series brings together professors, graduate students, and undergrads interested in condensed matter physics on a biweekly basis. We invite young faculty, postdocs, and advanced graduate students from the greater Boston area to come in, present their research, and chat with their peers here at Tufts. Speakers and attendees cover a range of disciplines including physics, chemistry, biophysics, engineering, and applied mathematics.

Spring 2019

Unless otherwise noted, Spring 2019 seminars are held on Wednesdays at 3:00pm - 4:00pm in Room 316 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.

February 6

An Eulerian method for mixed soft and rigid body interactions in fluids
Xiaolin Wang, Harvard University

Note location change: This seminar will be held in Room 204.

Abstract: Fluid-solid interaction problems are encountered in many engineering and biological applications, but are challenging to simulate due to the coupling between the two material phases. Typically, solids are simulated using a Lagrangian approach with a grid that moves with the material, whereas fluids are simulated using an Eulerian approach with a fixed spatial grid, requiring some type of interfacial coupling between the two different perspectives. Here, we present a fully Eulerian method for simulating structures immersed in fluids. By introducing a reference map variable to model finite-deformation constitutive relations in the structures on the same grid as the fluid, the interfacial coupling problem is highly simplified. The method is particularly well suited for simulating soft, highly-deformable materials and many-body contact problems. We also extend the technique to simulate rigid solids in an incompressible fluid, using a projection step formulated as a composite linear system that simultaneously enforces the rigidity and incompressibility constraints. Several examples including single deformable/rigid objects, multiple objects.

February 20

High-Throughput Simulations to Design Optimal Electrolytes for Energy Storage Devices
Nav Nidhi Rajput, Tufts University

Abstract:Today, the pursuit of transformative gains in the performance of electrical energy storage(EES) systems and industrial technologies are intrinsically a material’s problem, which requires the development of novel electrode materials, electrolytes, and architecture. Such innovations of novel electrodes as well as electrolytes for future EES systems lie inherently in the computationally driven design of materials by obtaining a fundamental understanding of the interplay between events scaling over wide spatial and temporal ranges. The composition of electrolytes has critical implications for the performance of current and future energy storage systems; from the formation and stability of the electrode-electrolyte interface to the transport properties, speciation, and viscosity of the bulk electrolyte. In this work, I present a high-throughput multi-scale modeling approach for screening salts and solvents important to multivalent (e.g., Mg2+, Ca2+ and Zn2+), chemical transformation (e.g., Li-S), and redox flow batteries. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode. We find that both Mg and Zn electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. Specifically, we observe that, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg2+→Mg+), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. We also study the effect of salt anion and the solvent on the solvation structure and dynamics of Li-polysulfide in the solution for application in Li-S battery. It is observed that the polysulfide chain length, solvent, and anions have a significant effect on the ion-ion and ion-solvent interaction as well as on the diffusion coefficient of the ionic species in solution. We also considered ionic liquid tethered ferrocene catholyte as redox center utilizing the Fc/Fc+ reaction for improving the performance of non-aqueous redox flow battery materials. It was observed that at solubility limit, the precipitation of solute is initiated through agglomeration of contact-ion pairs due to overlapping solvation shells. This works shows that the combination of modeling with experimental techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties of electrolytes, which is crucial in designing optimal electrolytes for beyond Li-ion batteries.

February 27

APS Talk Practice

Abstract: Students in condensed matter physics and related areas will practice their APS talks (may take two hours).

March 13

Swarms and Cells: Collective Behavior via Local Interactions in Biological Systems
Andrea Welsh, Georgia Tech

Abstract: Swarming is a ubiquitous self-organization phenomenon which occurs in many biological systems such as flocks of bird and insect, schools of fish, and collections of bacteria. This sort of behavior emerges spontaneously, arising without any sort of centralized control or leadership. Many crustaceans such as brine shrimp produce swarms in which individuals cluster together rather than spreading out uniformly in their environment. The size and distribution of these swarms are governed by local interactions between individuals. We will discuss the three-dimensional patterns that can be observed in brine shrimp swarms, specifically of the Great Salt Lake strain of Artemia franciscana, at high concentration. These patterns can be easily observed with simple tabletop experiments; however, the causes of these patterns are unknown. We experimentally test the effects of certain environmental conditions on the development of these swarms and discuss the mechanisms behind these swarms. Patterns can also develop in systems where individuals stay in fixed position and change state in time. This is true for systems like the cells in cardiac tissue where the local interactions between cells transfer electrical impulses and calcium ions. We particularly look at the spatial extensions of the FitzHugh-Nagumo model, a reaction diffusion system that is used to model cardiac cells and firing neurons like relaxation oscillators. We see the formation of two distinct types of dynamics despite oscillators being identical and locally coupled.

April 3

Title TBD
Michael Dimitriyev, Georgia Tech

April 10

Title TBD
Kerstin Nordstrom, Mount Holyoke College

April 17

Title TBD
Emily Davidson, Harvard University

April 24

Title TBD
Speaker TBD

Fall 2018

Unless otherwise noted, Fall 2018 seminars are held on Wednesdays at 3:00pm - 4:00pm in Room 316 of the Collaborative Learning and Innovation Complex (CLIC) - 574 Boston Avenue in Medford.

September 19

Research Soundbites

Abstract: Students in condensed matter physics and related areas will present short synopses of their current research projects.

September 26

Driving Forbidden Vibrational Overtones in Trapped Molecular Ions
David A. Hanneke - Amherst College

Abstract: Quantum control of atomic states has enabled understanding and tests of fundamental physics, produced incredibly accurate timekeeping, and advanced quantum computation and simulation. Molecules have extra degrees of freedom that bring both challenges and opportunities if they can be controlled at the quantum level. Active development of molecular control techniques has already produced impressive results. After a brief overview of the state of the art with small numbers of simple molecules, I will focus on one particular application: searches for new physics through time-variation of fundamental constants. Some models of quantum gravity and some classes of dark matter predict temporal changes in the proton-to-electron mass ratio. Molecular vibrations are sensitive to these changes. I will describe our work at Amherst with singly ionized oxygen molecules, both in a beam and a trap, and its prospects for testing physical laws.

October 3

Unexpected Tools for Programmed Properties and Switchable Fluorescence in Conjugated Molecules
Seth Sharber, Tufts University

Abstract: Advancing the frontiers of optoelectronic and responsive materials with conjugated molecules enables "bottom-up" control over material function from chemical structure, ideally where rational design of molecular components could dictate properties on-demand. However, predicting and directing properties in the solid state remains a central challenge due to the interplay of weak, non-covalent interactions that drive molecular conformation and assembly, which are integral to material properties but difficult to control. Our group has employed side chains, which traditionally function only to increase solubility, as non-covalent control units to achieve programmed assembly in conjugated materials, thereby tuning fluorescence and stimuli-responsive behavior in solids. Fluoroaromatic side chains in three-ring phenylene ethynylene (PE) oligomers introduce significant twisting into the conjugated backbone through fluoroarene-arene (ArF-ArH) interactions with the main chain, resulting in blue-shifted shifted optical properties compared to typical PE solids. Moreover, PEs with this twisted motif show mechanofluorochromic (MFC) response, in which mechanical force leads to a shift in fluorescence that is reversible with heat or solvent fuming. In this "side-chain engineering" domain, we have elucidated two unexpected tools that can tune optical and material properties. Terminal alkyl chains in the PE backbone can tune MFC response in PEs with green-to-orange and blue-to-green force-induced transitions. In addition, the regiochemical arrangement of fluorine atoms in the side chains significantly alters fluorescence and can invert the direction of the MFC response. These tools do not impact electronics of the system, but crystal structures show clear effects on molecular assembly with direct consequences for solid-state behavior, lending fundamental insight to these supramolecular structure-property relationships.

October 17

HIP Materials: Harnessing Interfacial Phenomena to Design New Soft Materials
Laura Bradley, UMass Amherst

Abstract: Fluid interfaces are fundamentally unique environments for materials synthesis and chemical operations that form the core of this exciting research. For example, interfacial mass transport provides avenues for exploitation in catalysis and separations. Interfacial structure and composition dictate the interactions between phases, giving rise to soft materials such as foams and emulsions. Interfaces are inherently open systems, in contact with bulk phases. Delivery from these phases provides versatile routes to engineer functional materials at fluid interfaces.

In gas—liquid interface engineering, deposition from the vapor-phase provides an important new path for materials synthesis. Here, this concept is explored and developed for vapor-phase deposition of functional polymers onto liquid substrates. A variety of morphologies are formed, ranging from particles, to planar films, to microstructured films, depending on wetting properties and polymer growth rates. Delivery and reaction of precursors in the bulk liquid provide additional degrees of freedom enabling formation of polymer—liquid gels, layered composite films, and encapsulating shells.

Materials design can also be advanced by imparting functionality to interfaces through structured colloids. Amphiphilic Janus particles of various shape have long been endorsed in this context as colloidal analogues to molecular surfactants. However, their widespread utilization has been impeded by the absence of scalable synthesis methods suitable for diverse chemistries and shapes. Here, I describe a newly developed synthesis platform to address this need: Clickable Janus particles that can be modified through thiol-yne click reactions with commercially available thiols which widen the palate of chemical compositions. The extent of modification can be used to control the particle morphology and thus the type of emulsion stabilized. I demonstrate the versatility of this method for generating Janus particles with pH- responsive properties to control emulsions in a tunable fashion.

October 31

Engineering Defects in Smectic Liquid Crystals: A Toolkit to Direct Assembly of Advanced Materials
Mohamed Gharbi, UMass Boston

Abstract: The opportunities for guiding assembly using energy stored in soft materials are wide open. The emerging scientific frontiers in this field show an exceptional promise for significant new applications. Since soft materials can be readily reconfigured, there are unplumbed opportunities to make responsive devices with tunable properties. Liquid crystals (LCs) constitute a fascinating class of matter characterized by the counterintuitive combination of fluidity and long-range order. These materials are known for their exceptionally successful applications in displays, smart windows, and biosensing applications. When the order of molecules in some local regions of LCs is not well defined, topological defects form. These defects are strong trapping sites for colloidal inclusions and have been widely used to control their assembly. In this talk, I will focus on defects in smectic liquid crystals: the focal conic domains (FCDs). I will show how these defects self-assemble into highly ordered structures named the "flower textures", when created on curved interfaces. These patterns are capable of focusing light and act as micro-lenses similar to an insect's compound eye. I will present recent progress in understanding the mechanisms that govern the formation of theses assemblies and will report how defects can inspire strategies for making a new generation of advanced materials.

November 7

Fast-moving bacteria self-organize into active two-dimensional crystals of rotating cells
Alexander Petroff, Clark University

Abstract: We investigate a new form of collective dynamics displayed by Thiovulum majus, one of the fastest-swimming bacteria known. Cells spontaneously organize on a surface into a visually striking two-dimensional hexagonal lattice of rotating cells. As each constituent cell rotates its flagella, it creates a tornado like flow that pulls neighboring cells towards and around it. In the first part of the talk, we describe the earliest stage of crystallization, the attraction of two bacteria into a hydrodynamically-bound dimer. In the second part of the talk, we present the dynamics of bacterial crystals, which are composed of 5--200 hydrodynamically bound cells. As cells rotate against their neighbors, they exert forces on one another, causing the crystal to rotate and cells to reorganize. We show how these dynamics arise from hydrodynamic and steric interactions between cells. We derive the equations of motion for a crystal, show that this model explains several aspects of the observed dynamics, and discuss the stability of these active crystals.

December 5

Engineered biomaterials to improve human health
Gulden Camci-Unal, UMass Lowell

Abstract: Regeneration of tissues that are damaged due to disease or trauma represents a major medical need. Although surgical replacement can be performed to address this issue, insufficient number of donors limits the applicability of the approach. There is an unmet demand for development of tissue replacements. My research aims to control and modulate cellular behavior for directing repair and regeneration of tissues. To achieve this goal, I use diverse tools from chemistry, cell biology, materials science, and engineering. In my seminar, I will talk about new biomaterial platforms to generate multicellular and compartmentalized tissue-mimetics for clinical applications including endothelialization of cardiovascular tissues, regeneration of bone, and invasion of tumors. To overcome the limitations with the conventional methods, we have developed novel approaches to assemble tissue-like structures. This strategy offers unique opportunities ranging from understanding fundamental biology to development of disease models for personalized medicine and organ assembly. The ultimate goal of my research is to improve human health and quality of life.